The present invention relates generally to certain compounds having optical properties. More particularly, as an example, the invention provides a specific compound comprising AxM(1-x)Al3B4O12 for use with selected wavelengths of electromagnetic radiation. x is larger than or equal to zero and smaller than or equal to 0.1, A is selected from a group consisting of Sc, Y, La, Yb, and Lu, and M is selected from a group consisting of Sc, Y, La, Yb, and Lu. Merely by way of example, the compound is useful for electromagnetic radiation having a wavelength of 350 nm and less, but it would be recognized that the invention has a much broader range of applicability.
Nonlinear optical (NLO) materials are unusual in that they affect the properties of light. A well-known example is the polarization of light by certain materials, such as when materials rotate the polarization vectors of absorbed light. If the effect on the polarization vector by the absorbed light is linear, then light emitted by the material has the same frequency as the absorbed light. NLO materials affect the polarization vector of the absorbed light in a nonlinear manner. As a result, the frequency of the light emitted by a nonlinear optical material is affected.
For example, when a beam of coherent light of a given frequency, such as produced by a laser, propagates through a properly oriented NLO crystal having non-zero components of the second order polarizability tensor, the crystal will generate light at a different frequency, thus extending the useful frequency range of the laser. Generation of this light can be ascribed to processes such as sum-frequency generation (SFG), difference-frequency generation (DFG) and optical parametric amplification (OPA). Devices using NLO crystals include, but are not limited to up and down frequency converters, optical parametric oscillators, optical rectifiers, and optical switches.
Frequency generation in NLO materials is usually an important effect. For example, two monochromatic electromagnetic waves with frequencies ω1 and ω2 propagating through a properly oriented NLO crystal can result in generation of light at a variety of frequencies. Mechanisms defining the frequency of light using these two separate frequencies are sum-frequency generation (SFG) and difference-frequency generation (DFG). SFG is a process where light of frequency ω3 is generated as the sum of the two incident frequencies, ω3=ω1+ω2. In other words, SFG is useful for converting long wavelength light to shorter wavelength light (e.g. near infrared to visible, or visible to ultraviolet). A special case of sum-frequency generation is second-harmonic generation (SHG) where ω3=2ω1, which is satisfied when the incident frequencies are equal, ω1=ω2. DFG is a process where light of frequency ω4 is generated as the difference of the incident frequencies ω4=ω1−ω2. DFG is useful for converting shorter wavelength light to longer wavelength light (e.g. visible to infrared). A special case of DFG is when ω1=ω2, hence ω4=0, which is known as optical rectification. Optical parametric oscillation (OPO) is also a form of DFG and is used to produce light at tunable frequencies.
The conversion efficiency of an NLO crystal for a particular application is dependent on a number of factors that include, but are not limited to: the effective nonlinearity of the crystal (picometers/volt [pm/V]), birefringence (Δn, where n is a refractive index), phase-matching conditions (Type I, Type II, non-critical, quasi, or critical), angular acceptance angle (radian-cm), temperature acceptance (.degree. K-cm), walk-off (radian), temperature dependent change in refractive index (dn/dT), optical transparency range (nm), and the optical damage threshold (W/cm2). Desirable NLO crystals should possess an optimum combination of the above properties as defined by the specific application.
Borate crystals form a large group of inorganic NLO materials used in various applications, such as laser-based manufacturing, medicine, hardware and instrumentation, communications, and research studies. Beta barium borate (BBO: β-BaB2O4), lithium triborate (LBO: LiB3O5), and cesium lithium borate (CLBO: CsLi(B3O5)2) are examples of borate-based NLO crystals developed in recent years that are being used widely as NLO devices, especially in high power applications. Select properties suitable for generation of laser light from the mid-infrared to the ultraviolet for these crystals are listed in Table 1.
TABLE 1Commercially Available NLO Materials and PropertiesPROPERTYBBOLBOCLBODeff (pm/V)2.20.80.9Optical Transmission (nm)190-3500160-2600180-2750Angular Acceptance (mrad-cm)0.86.50.6Temperature Acceptance (K-cm)557.52.5Walk-off Angle (deg.)30.61.8Damage Threshold (GW/cm2)51010Crystal Growth Propertiesflux or congr.fluxcongruent
BBO has a favorable non-linearity (about 2.2 pm/V), transparency between 190 nm and 3500 nm, significant birefringence (necessary for phase matching), and a high damage threshold (5 GW/cm2, 1064 nm, 0.1 ns pulse width). However, its high birefringence creates a relatively small angular acceptance that can limit conversion efficiencies and laser beam quality. The crystal is relatively difficult to grow to large sizes and is somewhat hygroscopic.
LBO exhibits optical transparency throughout the visible electromagnetic spectrum, extending well into the ultraviolet (absorption edge.congruent.160 nm), and possesses a high damage threshold (10 GW/cm2, 1064 nm, 0.1 ns pulse width). However, it has insufficient intrinsic birefringence for phase matching to generate deep UV radiation. Furthermore, LBO melts incongruently and must be prepared by flux-assisted crystal growth methods. This limits production efficiency that leads to small crystals and higher production costs.
CLBO appears capable of producing UV light due to a combination of high nonlinearity and sufficient birefringence. The crystal can also be manufactured to relatively large dimensions. However, the crystal usually is exceedingly moisture sensitive and often invariably sorbs water from the air; hence, extreme care usually must be taken to manage environmental moisture to prevent hydration stresses and possible crystal destruction.
In 1981 a crystal called NYAB [NdxY(1-x)Al3B4O12] was reported in the USSR. A laser self-frequency-doubling effect from 1320 nm to 660 nm was realized in a Nd0.2Y0.8Al3B4O12 crystal, but it was found that intrinsic crystal absorption at the second harmonic limited practical use of laser self-frequency-doubling from 1060 nm to 530 nm.
Years later several institutes in China succeeded in improving the crystal growing process and obtained NYAB crystals of good optical quality and reasonable size. Lu et al. developed a multi-functional crystal NdxY(1-x)Al3B4O12 with effective laser self-frequency-doubling conversion. The Nd3+ doped laser gain crystal was pumped with a dye laser, with laser emission at 1060 nm that was then converted to 530 nm within itself (see FIG. 2 of Lu et al., Chinese Phys. Lett. Vol. 3, No. 9 (1986)). NYAB has since been used as a research crystal that often is useful only in the visible spectrum. Recent work with Yb-doped YAB as a self-doubling laser gain material follows the same path as NYAB with small alterations in operational laser efficiency and wavelengths. Laser light is generated within the crystal and self-doubled into green 520 nm. (see Dekker et al., JOSA B, Vol. 22, No. 2 (2005) 378-384). Again, its operation and the historic method of preparation limit its use to the visible and infrared. Hence, it is highly desirable to improve techniques for this family of compounds that enable optical function down into the ultraviolet.
Hence, it is highly desirable to improve techniques for optical compounds.